This invention relates to an expansion tank for use with a hydrostatic transaxle and more particularly an expansion tank formed internally to the transaxle.
Hydrostatic transaxles are known in the art and generally comprise a hydrostatic transmission in the same housing as output gearing and/or axles. U.S. Pat. No. 5,314,387 discloses a known integrated hydrostatic transaxle (“IHT”) design, and shows a hydraulic pump receiving an input from a prime mover, hydraulic porting formed in a center section and a hydraulic motor connected to the pump through the porting. The motor drives an output shaft connected through gearing to a differential and output axles. Other transaxle designs are known in the art and can be used with this invention.
It is known to use an expansion tank with a hydrostatic transmission or IHT. One known design is to use an external tank that may be formed separate from the IHT housing or integrally therewith. An example of the latter design is shown in U.S. Pat. No. 5,314,387, which shows an expansion tank integrally formed as part of the upper housing, into which oil would expand as it heated, and which would then drain oil therefrom as it cooled. Such an expansion chamber would inherently be open to the atmosphere in order to work properly. Such prior art designs also use a siphon to move oil between the external expansion tank and the main sump. For example, the '387 patent also discloses mounting the expansion tank along the lower housing outside the differential gear and, given the level of oil in the main sump for the unit to operate properly, an expansion tank located at such a point would necessarily require the use of a siphon.
A further known design is shown in FIGS. 1 and 2, which show an integrated hydrostatic transaxle using an external expansion tank. This design was sold as a Model 310-3000 IHT, similar to that shown in U.S. Pat. No. 5,613,409, and therefore the internal workings of this IHT will not be discussed in detail. A main sump is formed inside the two primary casing portions 21 and 22 to retain the oil used by the hydrostatic transmission (not shown) and the gearing and differential (not shown), all of which are mounted internal to casings 21 and 22 and which drive output axles 23A and 23B.
An external expansion tank 25 is mounted on main casing 21 and secured thereto by means of a retention spring 26. A tube 28 in connected to main casing 21 through connector 29 and to expansion tank 25 through connector 27, with connectors 27 and 29 being right angle fittings with SAE straight thread O-ring and hose barb ends. Since the level of the expansion tank 25 is below that of the oil in the main sump, tube 28 forms a vacuum siphon with casings 21 and 22 to transfer oil to and from the expansion tank 25. As the oil is heated during operation of the IHT it would expand through tube 28 into expansion tank 25. As the oil cooled and contracted, oil would be forced from expansion tank 25 back into the main sump through tube 28 by a vacuum siphon effect. Air leaves or enters expansion tank 25 by means of breather vent 24 as oil expands into or contracts from expansion tank 25. Such a design has several drawbacks, however, including the need for locating and attaching a separate molded expansion tank on the outside of the housing, which increases costs and can present clearance issues and similar problems.
Another known design is the use of an internal expandable bellows, such as is shown in U.S. Pat. No. 4,987,796. As the oil pressure on the bellows changes due to expansion, the bellows collapses or enlarges the main sump area accordingly. This design also has significant drawbacks, including problems with maintaining flexibility of the bellows and the increased costs in adding such a unit to the IHT. Furthermore, if the bellows fails, the unit will leak oil, which may be hot, out of the transmission casing and will ultimately fail.